One of the critical parameters involved in the manufacture of paper is its strength. Virtually all paper manufactured is sold with a strength specification of some sort, but up to the present time it has not been practical to accurately measure the strength of paper "on line" as it is being manufactured. Since paper making is a high speed continuous process, large amounts of paper can easily be produced before the strength of the paper made can be confirmed by subsequent measurement.
Strength specifications for paper are usually given in terms of an empirical destructive test, one of the more common of these being the "burst pressure" or "Mullen" test. A burst pressure test is conducted by clamping a sample of the paper between two circular clamping rings having a specified standard inside diameter, and building up pressure on one side of the paper until the paper bursts (using a rubber diaphragm and liquid pressure). The pressure required to burst the paper is known as the "burst pressure" and is the figure often used to specify the required strength.
Needless to say, the burst pressure test does not lend itself for use in connection with the continuous measurement of paper strength. Because of its widespread popularity, however, any other method used to measure the strength of paper should provide results which correlate with the standard burst pressure test.